Scientists from Brazilian institutions, in collaboration with researchers from Israel, “manipulated” carbon nanotubes of 1 nm diameter deposited on quartz surfaces and analyzed strain and displacements produced by this nanointervention. The team identified some behavior patterns in the nanotubes – quartz system and formulated a mathematical model applicable to systems formed by one- and two-dimensional materials over various substrates. The results of the study were recently published in Nano Letters.

To perform the experiments, the Brazilian investigators used samples idealized and produced in the Weizmann Institute of Science (Israel), in which the nanotubes are serpentine-shaped (composed of parallel segments connected together by U-shaped curves).These samples offered a desirable complexity, fostered by both the nanotubes format and the anisotropic character of quartz, which makes adhesion of nanotubes to the substrate not the same at all points.

In order to “manipulate” the system, the researchers used the tip of an atomic force microscope (AFM) built in the laboratory, which allows to change the position of nanometric particles and even of atoms, and to measure in situ the optical spectrum of nanostructures. In each sample, the tip touched a point of the quartz substrate and pushed toward the nanotube, and then proceeded to the optical analysis.

Before and after nanomanipulation, the scientists analyzed a number of points in the nanotube using the technique of Raman spectroscopy, which provides information about the frequency in which the atoms vibrate in the area being studied. More specifically, researchers focused on the frequency of the “G band”, which is used to infer the strain measurements of a considered point, since changes in the frequency of the “G band” are proportional to changes in strain.

Thus, scientists were able to identify and analyze different behavior of the nanotubes after nanomanipulation; for example, the detachment of the substrate and the intense displacement of a full stretch of the nanotube that had received two manipulations at the same point.

In addition to performing the experimental work, the authors of the article in Nano Letters managed to condense the complexity of behaviors they observed in a mathematical model (an equation) capable of explaining them theoretically and predicting these phenomena in similar systems. “The paper proposes a relatively simple model to describe complex effects of nanostructures adhesion in support media,” says Ado Jório, professor in the Department of Physics of the Federal University of Minas Gerais (UFMG) signing the letter as corresponding author.

The research that led to the Nano Letters article was developed within the master’s, doctoral and postdoctoral work of three authors of the letter, in the context of the Brazilian Network for Research and Instrumentation in Optical Nano-Spectroscopy, a project funded by the National Council for Scientific and Technological Development (CNPq) and coordinated by Ado Jório. “This is the result of a broad scientific instrumentation project, which aims at reaching the level of manipulating nanostructures and measuring, accurately, the effect of this process at the nanoscale,” says Jório.

The figure shows one of the 34serpentine-shaped nanotubes on crystalline quartz substrate studied by the authors of the article. To the left of the reader is the nanotube before manipulation. To the right, following the sequence, the same nanotube after the intervention, with the consequent evident strain. The central segment of the nanotube, where the nanomanipulation occurred, was colorized, the gray scale indicating the frequency of the G band in that place. Finally, farther to the right, the chart displays the frequency of G band measured by Raman spectroscopy in successive points of this nanotube (graphical representation of gray hues): the black circles refer to non-manipulated nanotube and the gray colored circles, to the manipulated ones.

With an innovative approach on an academic and industrial problem, a study wholly conducted in Brazil has brought significant advances in the understanding of the adhesion of DLC (diamond-like carbon) films on steels. The results of the work, which were recently published in the journal Applied Materials and Interfaces of the American Chemical Society (ACS), can help optimize such adhesion, thus prolonging the life of DLC films and expanding their use in the industry.

The team of scientists was particularly interested in the DLC potential to increase the energy efficiency of internal combustion engines. In fact, if all car engine components were coated with DLC films, the owner of that car would spend 5-10% less fuel and save the environment a good deal of greenhouse gas emissions and other pollutants, among other advantages. The reason for such saving lies in the ultra-low friction of DLC, since friction is the force responsible for wasting fuel while providing resistance to the motion that the parts of the engine make among themselves.

However, DLC has a drawback: it does not adhere to steel, causing quick delamination of the films from the substrate. To work around this problem, both in the laboratory and in industry, it is customary to deposit a layer containing silicon, known as interlayer, over the steel. The DLC film is then deposited on top of it. The result is a “sandwich”, which does not come undone easily.

In the paper published in the ACS journal, the authors experimentally analyzed a “sandwich” consisting of a steel substrate, an interlayer of silicon carbide (SiC) and a DLC film. Both the interlayer and the film were deposited by a quick process that generated thin layers of a few nanometers (up to 10). Mainly, two issues differentiated this study from other similar studies in the scientific literature. Firstly, the team focused in analyzing what happened in two regions corresponding to the interfaces of the interlayer with the film (upper) and with the steel (lower). Secondly, the scientists made a chemical approach on the matter of adhesion.

“This work has identified the chemical structure that generates adhesion in lower (SiCx: H/steel) and upper (a-C:H/SiCx:H) interfaces, which make up the a-C:H/SiCx:H/steel sandwich structure”, said Carlos A. Figueroa, professor at the University of Caxias do Sul (UCS) and corresponding author of the article. “The mechanisms found in the bibliography raised physical or mechanical aspects, but not chemical ones,” said Figueroa, who graduated in chemical sciences from the University of Buenos Aires (UBA) and has a doctorate degree in physics from the State University of Campinas (Unicamp). “However, adhesion is generated by the sum of all individual chemical bonds existing among DLC, the interlayer and steel,” he adds.

Scientists kept a constant film deposition temperature, but varied the interlayer deposition temperature, generating a group of samples deposited at 100° C and another one at over 300° C. After analyzing them by a variety of techniques, especially, X-ray photoelectron spectroscopy (XPS), researchers found that the lower interface of the interlayer, regardless of the deposition temperature, was largely composed of silicon atoms (from the interlayer) bonded to iron atoms (from the substrate). At the upper interface of the interlayer, the team found differences according to the deposition temperature of the interlayer. In the samples deposited at 100° C, oxygen atoms bonded many of the silicon and carbon atoms, preventing the carbon of the film to strongly bond to the silicon of the interlayer, and resulting in a film without good adhesion. In turn, scientists did not find oxygen in the interface of the samples deposited at over 300° C, but bonds between carbon and silicon atoms, which caused the film adhere well to the interlayer.

Schematic illustration of the chemical bonds present in the upper and lower interfaces of the interlayer deposited at 100° C (left) and over 300° C (right). In the center, a real engine cylinder displays, on the left side, a DLC film (in black) delaminated on the interlayer deposited at 100° C and, in the right side, the same film well adhered on the interlayer deposited over 300° C.

Besides Figueroa and students of the research group he leads in UCS, also authored the paper researchers from the Institute of Physics at Unicamp, where the XPS measures were made, as well as a scientist from the Federal University of Rio Grande do Sul (UFRGS) that, together with the other authors, participated in the discussion of results.

The work received the support from Brazilian Science funding bodies (Capes, CNPq through INCT National Institute of Surface Engineering, Fapergs), of Petrobras, UCS, the European Commission (Marie Skłodowska – Curie Actions) and Plasmar Tecnologia (a small company that is developing, through a TECNOVA RS project, an industrial equipment to deposit DLC on steel aiming to increase the energy efficiency of car engines).